Network Working Group T. George
Request for Comments: 4165 B. Bidulock
Category: Standards Track OpenSS7
R. Dantu
University of North Texas
H. Schwarzbauer
Siemens
K. Morneault
Cisco Systems
September 2005
Signaling System 7 (SS7) Message Transfer Part 2 (MTP2) -User Peer-to-Peer Adaptation Layer (M2PA)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines a protocol supporting the transport of
Signaling System Number 7 (SS7) Message Transfer Part (MTP) Level 3
signaling messages over Internet Protocol (IP) using the services of
the Stream Control Transmission Protocol (SCTP). This protocol would
be used between SS7 Signaling Points using the MTP Level 3 protocol.
The SS7 Signaling Points may also use standard SS7 links using the
SS7 MTP Level 2 to provide transport of MTP Level 3 signaling
messages. The protocol operates in a manner similar to MTP Level 2
so as to provide peer-to-peer communication between SS7 endpoints.
George, et al. Standards Track [Page 1]

RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 20051. Introduction1.1. Scope
There is a need for Switched Circuit Network (SCN) signaling protocol
delivery over an IP network. This includes message transfer between
the following:
- a Signaling Gateway (SG) and a Media Gateway Controller (MGC)
[RFC2719]
- a SG and an IP Signaling Point (IPSP)
- an IPSP and an IPSP
This could allow for convergence of some signaling and data networks.
SCN signaling nodes would have access to databases and other devices
in the IP network domain that do not use SS7 signaling links.
Likewise, IP telephony applications would have access to SS7
services. There may also be operational cost and performance
advantages when traditional signaling links are replaced by IP
network "connections".
The delivery mechanism described in this document allows for full
MTP3 message handling and network management capabilities between any
two SS7 nodes communicating over an IP network. An SS7 node equipped
with an IP network connection is called an IP Signaling Point (IPSP).
The IPSPs function as traditional SS7 nodes using the IP network
instead of SS7 links.
The delivery mechanism should:
- Support seamless operation of MTP3 protocol peers over an IP
network connection.
- Support the MTP Level 2 / MTP Level 3 interface boundary.
- Support management of SCTP transport associations and traffic
instead of MTP2 Links.
- Support asynchronous reporting of status changes to management.
1.2. Terminology
MTP - The Message Transfer Part of the SS7 protocol [Q.700] [Q.701]
[Q.702] [Q.703] [Q.704] [Q.705] [T1.111].
MTP2 - MTP Level 2, the MTP signaling link layer.
George, et al. Standards Track [Page 3]

RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
MTP3 - MTP Level 3, the MTP signaling network layer.
MTP2-User - A protocol that normally uses the services of MTP Level
2. The only MTP2 user is MTP3. The MTP2 user is equivalent to the
M2PA user.
Signaling End Point (SEP) - An SS7 Signaling Point that originates or
terminates signaling messages. One example is a central office
switch. [RFC2719]
IP Signaling Point (IPSP) - An SS7 Signaling Point with an IP network
connection used for SS7 over IP.
Signaling Gateway (SG) - A signaling agent that receives/sends SCN
native signaling at the edge of the IP network [RFC2719]. In this
context, an SG is an SS7 Signaling Point that has both an IP network
connection used for SS7 over IP, and a traditional (non-IP) link to
an SS7 network.
Signal Transfer Point (STP) - A Signal Transfer Point as defined by
MTP standards, e.g., [Q.700].
Signaling Point (STP) - A Signaling Point as defined by MTP
standards, e.g., [Q.700].
Association - An association refers to an SCTP association [RFC2960].
The association provides the transport for MTP3 protocol data units
and M2PA adaptation layer peer messages.
Network Byte Order - Most significant byte first, also known as "Big
Endian". See [RFC791], Appendix B "Data Transmission Order".
Stream - A stream refers to an SCTP stream [RFC2960].
1.3. Abbreviations
BSNT - Backward Sequence Number to be Transmitted
FSNC - Forward Sequence Number of last message accepted by remote
level 2
LI - Length Indicator
MSU - Message Signal Unit
SCCP - Signaling Connection Control Part
SCN - Switched Circuit Network
George, et al. Standards Track [Page 4]

RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
SCTP - Stream Control Transmission Protocol
SIF - Signaling Information Field
SIO - Service Information Octet
SLC - Signaling Link Code
SS7 - Signaling System Number 7
SSN - Stream Sequence Number
STP - Signal Transfer Point
1.4. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.5. Signaling Transport Architecture
The architecture that has been defined [RFC2719] for Switched Circuit
Network (SCN) signaling transport over IP uses multiple components,
including an IP transport protocol, the Stream Control Transmission
Protocol (SCTP), and an adaptation module to support the services
expected by a particular SCN signaling protocol from its underlying
protocol layer.
Within this framework architecture, this document defines an SCN
adaptation module that is suitable for the transport of SS7 MTP3
messages. The adaptation layer, known as the MTP2 User Peer-to-peer
Adaptation Layer (M2PA), provides MTP3 with an interface and services
similar to MTP2. In effect, MTP2 and lower layers of the traditional
SS7 protocol stack are replaced by an IP equivalent.
Figure 1 shows the seamless interworking at the MTP3 layer. MTP3 is
adapted to the SCTP layer using the MTP2 User Peer-to-peer Adaptation
Layer (M2PA). All the primitives between MTP3 and MTP2 are supported
by M2PA. The SCTP association acts as one SS7 link between the
IPSPs. An IPSP may have the Signaling Connection Control Part (SCCP)
and other SS7 layers above MTP3.
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
M2PA receives the primitives sent from MTP3 to its lower layer. M2PA
processes these primitives or maps them to appropriate primitives at
the M2PA/SCTP interface. Likewise, M2PA sends primitives to MTP3
similar to those used in the MTP3/MTP2 interface.
Because M2PA uses larger sequence numbers than MTP2, the MTP3
Changeover procedure MUST use the Extended Changeover Order and
Extended Changeover Acknowledgement messages described in [Q.2210]
and [T1.111].
Also, the following MTP3/MTP2 primitives must use the larger sequence
numbers:
- BSNT Confirmation
- Retrieval Request and FSNC
1.6.2. Support for Peer-to-Peer Communication
In SS7, MTP Level 2 sends three types of messages, known as signal
units: Message Signal Units (MSUs), Link Status Signal Units (LSSUs),
and Fill-In Signal Units (FISUs).
MSUs originate at a higher level than MTP2, and are destined for a
peer at another node. Likewise, M2PA passes these messages from MTP3
to SCTP as data for transport across a link. These are called User
Data messages in M2PA.
LSSUs allow peer MTP2 layers to exchange status information.
Analogous messages are needed for M2PA. The Link Status message
serves this purpose.
FISUs are transmitted continuously when no other signal units are
waiting to be sent. FISUs also carry acknowledgement of messages.
Since an IP network is a shared resource, it would be undesirable to
have a message type that is sent continuously as is the case with
FISUs. Furthermore, SCTP does not require its upper layer to
continuously transmit messages. Therefore, M2PA does not provide a
protocol data unit like the FISU. The M2PA User Data message is used
to carry acknowledgement of messages. If M2PA needs to acknowledge a
message, and it has no MTP3 message of its own to send, an empty User
Data message can be sent.
George, et al. Standards Track [Page 8]

RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 20051.7. Functions Provided by M2PA1.7.1. MTP2 Functionality
M2PA provides MTP2 functionality that is not provided by SCTP; thus,
together M2PA and SCTP provide functionality similar to that of MTP2.
SCTP provides reliable, sequenced delivery of messages.
M2PA functionality includes:
- Data retrieval to support the MTP3 changeover procedure
- Reporting of link status changes to MTP3
- Processor outage procedure
- Link alignment procedure
1.7.2. Mapping of SS7 and IP Entities
The M2PA layer must maintain a map of each of its SS7 links to the
corresponding SCTP association.
1.7.3. SCTP Association Management
SCTP allows a user-specified number of streams to be opened during
the initialization. It is the responsibility of the M2PA layer to
ensure proper management of the streams allowed within each
association.
M2PA uses two streams in each direction for each association. Stream
0 in each direction is designated for Link Status messages. Stream 1
is designated for User Data messages, as well as Link Status messages
that must remain in sequence with the User Data messages. Separating
the Link Status and User Data messages into separate streams allows
M2PA to prioritize the messages in a manner similar to MTP2.
Notifications received from SCTP are processed by M2PA or translated
into an appropriate notification to be sent to the upper layer MTP3.
1.7.4. Retention of MTP3 in the SS7 Network
M2PA allows MTP3 to perform all of its Message Handling and Network
Management functions with IPSPs as it does with other SS7 nodes.
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
The Data field contains the following fields of the MTP Message
Signal Unit (MSU):
- the Message Priority field (PRI)
- Service Information Octet (SIO)
- Signaling Information Field (SIF)
The MTP MSU is described in Q.703 [Q.703], Section 2.2, "Signal Unit
Format", and T1.111.3 [T1.111], Section 2.2, "Signal Unit Format".
The Japanese TTC standard uses the PRI field as an MTP3 Message
Priority field [JT-Q703] [JT-Q704]. For versions of MTP that do not
use these two bits, the entire first octet of the Data field is
spare.
The format of the first octet of the Data field is:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|PRI| spare | (followed by SIO, SIF)
+-+-+-+-+-+-+-+-+
PRI - Priority used only in national MTP defined in [JT-Q703] and
[JT-Q704]. These bits are spare for other MTP versions.
Note that the Data field SHALL NOT contain other components of the
MTP MSU format:
- Flag
- Backward Sequence Number (BSN)
- Backward Indicator Bit (BIB)
- Forward Sequence Number (FSN)
- Forward Indicator Bit (FIB)
- Length Indicator (LI)
- Check bits (CK)
The Data field SHALL be transmitted in the byte order as defined by
MTP3.
M2PA SHALL NOT add padding to the MTP3 message.
Note: In the SS7 Recommendations, the format of the messages and
fields within the messages are based on bit transmission order. In
these recommendations, the Least Significant Bit (LSB) of each field
is positioned to the right. The received SS7 fields are populated
octet by octet as received into the 4-octet word, as shown below.
George, et al. Standards Track [Page 15]

RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 20052.3.2.1. Link Status Proving
The Link Status Proving message may optionally carry additional
bytes. If the optional bytes are used, the format of the message is
as follows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| State |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ filler /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
It is RECOMMENDED that the length of the Link Status Proving message
be similar to the size of the User Data messages that will be carried
on the link.
It is RECOMMENDED that the filler field contain a number pattern that
varies among the Link Status Proving messages, and that allows the
SCTP checksum [RFC3309] to be used to verify the accuracy of
transmission.
3. State Control3.1. SCTP Association State Control
Figure 7 illustrates state changes in the M2PA management of the SCTP
association, together with the causing events. Note that some of the
error conditions are not shown in the state diagram.
Following is a list of the M2PA Association States and a description
of each.
IDLE - State of the association during power-up initialization.
ASSOCIATING - M2PA is attempting to establish an SCTP association.
ESTABLISHED - SCTP association is established.
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 20054. Procedures
Because M2PA provides MTP3 with an interface and functionality like
MTP2, its internal functioning is similar to that of MTP2.
Except as modified in this document, M2PA SHOULD follow the
requirements of the applicable MTP2 specification. These may include
[Q.703] or [T1.111]. The same standard MUST be followed on both ends
of the M2PA link.
In particular, the corresponding applicable timer value defaults and
ranges specified for the applicable MTP2 standard should be used for
the M2PA timers.
When referring to MTP2 terminology in this document, the terminology
of [Q.703] is used. This does not imply that the requirements of
[Q.703] are to be followed.
4.1. Procedures to Support MTP2 Features4.1.1. Signal Unit Format, Delimitation, Acceptance
Messages for transmission across the network must follow the format
described in Section 2.
SCTP provides reliable, in-sequence delivery of user messages.
Therefore the related functionality of MTP2 is not needed. SCTP does
not provide functions related to Link State Control in MTP2. These
functions must be provided by M2PA.
Since SCTP provides delivery of messages, there is no need for M2PA
to delimit its messages with a flag, as is done in MTP2.
Furthermore, M2PA does not need to perform zero bit insertion and
deletion on its messages.
Since SCTP uses a checksum to detect transmission errors, there is no
need for an M2PA checksum, as is needed in MTP2. This also
eliminates the need for the error rate monitors of MTP2.
Since SCTP provides reliable delivery and ordered delivery, M2PA does
not perform retransmissions. This eliminates the need for the
forward and backward indicator bits in MTP2 signal units.
Acceptance of a message is indicated by a successful receipt of the
message from SCTP.
George, et al. Standards Track [Page 19]

RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 20054.1.2. MTP and SCTP Entities
This section describes how M2PA relates MTP and SCTP entities.
Each MTP link corresponds to an SCTP association. To prevent
duplicate associations from being established, it is RECOMMENDED that
each endpoint know the IP address (or IP addresses, if multi-homing
is used) and port number of both endpoints. SCTP prevents two
associations with the same IP addresses and port numbers from being
established.
It is necessary for at least one of the endpoints to be listening on
the port on which the other endpoint is trying to establish the
association. Therefore, at least one of the port numbers SHOULD be
the M2PA registered port.
If only one association is to be established between these two IP
addresses, then the association SHOULD be established using the M2PA
registered port at each endpoint.
If it is desirable to create multiple associations (for multiple
links) between the two IP addresses, different port numbers can be
used for each association. Nevertheless, the M2PA registered port
number SHOULD be used at one end of each association.
Each combination of IP address/port for the two endpoints (i.e., each
association) MUST be mapped to the same Signaling Link Code (SLC) at
each endpoint, so that each endpoint knows which link is being
created at the time the SCTP association is established. However,
M2PA does not do any processing based on the SLC.
Following are examples of the relationships between associations and
links. Note that a link is an SCTP association identified by two
endpoints. Each endpoint is identified by an IP address and port
number. Each association is mapped to an SLC.
Figure 8 shows a case with two IPSPs, each with two IP addresses.
Two associations are the links that connect the two IPSPs. Since
these links are in the same link set, they MUST have different SLCs.
Table 1 shows the relationships in tabular form. Table 1 is only
conceptual. The actual method for mapping the SCTP associations to
the SLCs is implementation dependent.
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
Link alignment takes place after the association is established. If
SCTP fails to establish the association, and M2PA has received a
Start Request from its MTP3, then M2PA SHALL report to MTP3 that the
link is out of service.
The Link Status Out of Service message replaces the SIOS message of
MTP2. Unlike MTP2, the message SHOULD NOT be transmitted
continuously. After the association is established, M2PA SHALL send
a Link Status Out of Service message to its peer. Prior to the
beginning of alignment, M2PA MAY send additional Link Status Out of
Service messages.
The Link Status Alignment message replaces the SIO message of MTP2.
This message is sent to signal the beginning of the alignment
procedure. The Link Status Alignment message SHOULD NOT be
transmitted continuously. M2PA MAY send additional Link Status
Alignment until it receives Link Status Alignment, Link Status
Proving Normal, or Link Status Proving Emergency from the peer.
The Link Status Proving Normal message replaces the SIN message of
MTP2. The Link Status Proving Emergency message replaces the SIE
message of MTP2.
The proving period MAY be omitted if this is allowed by the
applicable MTP2 standard (e.g., [Q.2140]).
If proving is performed, then during the proving period (i.e., after
M2PA starts the proving period timer T4), M2PA SHALL send Link Status
Proving messages to its peer at an interval defined by the protocol
parameter Proving_Interval. It is RECOMMENDED that Proving_Interval
be set so that the traffic load generated with the Link Status
Proving messages during the proving period is comparable to the
normal traffic load expected when the link is in service.
The Link Status Ready message replaces the FISU of MTP2 that is sent
at the end of the proving period. The Link Status Ready message is
used to verify that both ends have completed proving. When M2PA
starts timer T1, it SHALL send a Link Status Ready message to its
peer in the case where MTP2 would send a FISU after proving is
complete. If the Link Status Ready message is sent, then M2PA MAY
send additional Link Status Ready messages while timer T1 is running.
These Link Status Ready messages are sent on the Link Status stream.
In the case that MTP2 sends an MSU or SIPO message at the end of
proving, M2PA SHALL send (respectively) a User Data or Link Status
Processor Outage message.
George, et al. Standards Track [Page 25]

RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 20054.1.4. Processor Outage
The Link Status Processor Outage message replaces the SIPO message of
MTP2. Unlike MTP2, the message SHOULD NOT be transmitted
continuously. M2PA SHALL send a Link Status Processor Outage message
to its peer at the beginning of a processor outage condition where
MTP2 would send SIPO. M2PA MAY send additional Link Status Processor
Outage messages as long as that condition persists. The Link Status
Processor Outage message SHALL be sent on the User Data stream.
While in a local processor outage (LPO) condition:
(a) Any User Data messages received from the peer MUST NOT be
acknowledged and MUST be buffered.
(b) M2PA SHOULD continue to acknowledge User Data messages
received and accepted by MTP3 before the local processor
outage.
(c) M2PA SHOULD continue to transmit messages that have been sent
by its upper layer MTP3.
While there is a remote processor outage (RPO) condition:
(a) M2PA SHOULD continue to acknowledge User Data messages
received and accepted by MTP3, regardless of the remote
processor outage.
(b) If any User Data messages received from the peer after the
Link Status Processor Outage cannot be delivered to MTP3, then
these messages MUST NOT be acknowledged and MUST be buffered.
If M2PA receives a Flush command from MTP3,
(a) M2PA SHALL discard any incoming messages that were queued and
unacknowledged during the processor outage condition.
(b) M2PA SHALL discard messages in the transmit and retransmit
queues as required by MTP2.
If M2PA receives a Continue command from MTP3, M2PA SHALL begin
processing the incoming messages that were queued and unacknowledged
during the processor outage condition.
When the local processor outage condition ends, M2PA SHALL send a
Link Status Processor Recovered message to its peer on the User Data
stream. This message is used to signal the end of the processor
outage condition, instead of an MSU or FISU, as is used in MTP2. The
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
BSN in the Link Status Processor Recovered message is set to the FSN
of the last User Data message received (and not discarded) from the
peer M2PA. M2PA SHALL cease transmitting User Data messages after
sending the Link Status Processor Recovered message, until it has
received the Link Status Ready message (see below).
Upon receiving the Link Status Processor Recovered message, the M2PA
in RPO SHALL respond with a Link Status Ready message on the User
Data stream. The BSN in the Link Status Ready message is set to the
FSN of the last User Data message received (and not discarded) from
the peer M2PA.
Upon receiving the Link Status Ready message, the M2PA formerly in
LPO SHALL respond with a Link Status Ready message on the User Data
stream. The BSN in the Link Status Ready message is set to the FSN
of the last User Data message received (and not discarded) from the
peer M2PA.
M2PA (at both the LPO and RPO ends) uses the BSN value in the
received Link Status Ready message to resynchronize its sequence
numbers, if this is required by MTP2. M2PA SHALL NOT resume
transmitting User Data messages until it has sent the Link Status
Ready message.
During resynchronization, M2PA SHALL NOT discard any received User
Data messages that were sent after the processor outage ended.
When M2PA experiences a local processor outage, it MAY put the link
out of service by sending a Link Status Out of Service message, if
this is allowed by the applicable MTP2 standard (e.g., [Q.2140]).
In other respects, M2PA SHOULD follow the same procedures as MTP2 in
processor outage.
4.1.5. Level 2 Flow Control
The Link Status Busy message replaces the SIB message of MTP2. The
message SHOULD NOT be transmitted continuously. M2PA SHALL send a
Link Status Busy message to its peer at the beginning of a receive
congestion condition where MTP2 would send SIB. M2PA MAY send
additional Link Status Busy messages as long as that condition
persists. When the condition ends, M2PA SHALL send a Link Status
Busy Ended message to its peer.
M2PA SHALL continue transmitting messages while it is in receive
congestion, but MUST NOT acknowledge the message that triggered the
sending of the Link Status Busy message, nor any messages received
before the sending of Link Status Busy Ended.
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
When the peer M2PA receives the first Link Status Busy message, it
SHALL start the Remote Congestion timer T6 if there are messages in
the retransmission buffer awaiting acknowledgement (i.e., T7 is
running). M2PA SHALL stop the T7 timer if it is running. Additional
Link Status Busy messages received while T6 is running do not cause
T6 to be reset and do not cause T7 to be started. While T6 is
running, T7 SHALL NOT be started.
When the peer M2PA receives the Link Status Busy Ended message and T6
has not expired, it SHALL stop T6 (if T6 is running) and start T7 (if
there are messages awaiting acknowledgement in the retransmission
buffer).
The peer M2PA SHOULD continue receiving and acknowledging messages
while the other end is busy, but MUST NOT send User Data messages
after receiving Link Status Busy and before receiving Link Status
Busy Ended.
4.1.6. Link Out of Service
The Link Status Out of Service message replaces the SIOS message of
MTP2. Unlike MTP2, the message SHOULD NOT be transmitted
continuously. M2PA SHALL send a Link Status Out of Service message
to its peer at the beginning of a condition where MTP2 would send
SIOS. M2PA MAY send additional Link Status Out of Service messages
as long as that condition persists.
When M2PA places a link in the OUT OF SERVICE state, M2PA SHOULD NOT
terminate the SCTP association.
4.1.7. SCTP Association Problems
The SCTP association for a link may become unusable, such as when one
of the following occurs:
- SCTP sends a Send Failure notification to M2PA.
- SCTP sends a Communication Lost notification to M2PA.
- SCTP sends a Communication Error notification to M2PA.
- The SCTP association is lost.
If the SCTP association for a link becomes unable to transmit or
receive messages, M2PA SHALL report to MTP3 that the link is out of
service and enter the OUT OF SERVICE state.
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 20054.1.8. Transmission and Reception Priorities
In MTP, Link Status messages have priority over User Data messages
([Q.703], Section 11.2). To achieve this in M2PA, M2PA uses separate
streams in its SCTP association for Link Status messages and User
Data messages.
M2PA SHALL send all messages using the ordered delivery option of
SCTP.
M2PA SHOULD give higher priority to messages sent on the Link Status
stream than to messages sent on the User Data stream when sending
messages to SCTP.
M2PA SHOULD give higher priority to reading the Link Status stream
than to reading the User Data stream.
M2PA SHOULD give higher priority to receiving notifications from SCTP
than to reading either the Link Status stream or the User Data
stream.
4.1.9. M2PA Version Control
A node upgraded to a newer version of M2PA SHOULD support the older
versions used on other nodes with which it is communicating. If that
is the case, then alignment can proceed normally.
In particular, it is recommended that for future modifications to
this protocol:
- Any newer version SHOULD be able to process the messages from an
older version.
- A newer version of M2PA SHOULD refrain from sending messages to
an older version of M2PA messages that the older version cannot
process.
- If an older version of M2PA receives a message that it cannot
process, it SHOULD discard the message.
- In cases where different processing is done in two versions for
the same format of a message, then the newer version SHOULD
contain procedures to recognize and handle this appropriately.
In case a newer version of M2PA is incompatible with an older
version, the newer version SHOULD recognize this and prevent the
alignment of the link. If a Link Status Alignment message with an
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
unsupported version is received by the newer version, the receiving
end's M2PA SHOULD reply with a Link Status Out of Service message and
not complete the alignment procedure.
4.2. Procedures to Support the MTP3/MTP2 Interface4.2.1. Sending and Receiving Messages
When MTP3 sends a message for transmission to M2PA, M2PA passes the
corresponding M2PA message to SCTP using the SEND primitive.
User Data messages SHALL be sent via the User Data stream (stream 1)
of the association.
M2PA Link Status messages are passed to SCTP using the SEND
primitive.
The following Link Status messages SHALL be sent on the Link Status
stream (stream 0):
- Alignment
- Proving Normal
- Proving Emergency
- Ready (when sent during alignment)
- Busy
- Busy Ended
- Out of Service
The following Link Status messages SHALL be sent on the User Data
stream (stream 1):
- Processor Outage
- Processor Recovered
- Ready (when sent at the end of processor outage)
If M2PA receives a message from SCTP with an invalid Message Class or
unsupported Message Type in the Common Message Header, M2PA SHALL
discard the message.
For message types other than User Data, the Forward Sequence Number
is set to the FSN of the last User Data message sent.
If M2PA receives a User Data message with an FSN that is out of
order, M2PA SHALL discard the message.
Note: In all calculations involving FSN and BSN, the programmer
should be aware that the value wraps around to 0 after reaching its
maximum value.
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
When there is a message to acknowledge, M2PA MUST acknowledge the
message with the next User Data message sent. If there is no User
Data message available to be sent when there is a message to
acknowledge, M2PA SHOULD generate and send a User Data message with
no data payload, without delay. (In other words, in the case where
MTP2 would acknowledge a message with a FISU, M2PA SHOULD acknowledge
the message with an empty User Data message.) The FSN for this empty
User Data message is not incremented. It MUST contain the same FSN
as the most recently sent User Data message that contains data.
Delaying of acknowledgements can result in poor SS7 performance.
If M2PA receives an empty User Data message, it SHALL NOT send an
acknowledgement of that message.
Note that there is no reason to place Link Status messages or empty
User Data messages in the M2PA retransmit buffer, since these
messages are not retrieved for changeover and timer T7 does not apply
to them.
Note that since SCTP provides reliable delivery and ordered delivery
within the stream, M2PA does not perform retransmissions.
Nevertheless, M2PA SHALL retain transmitted User Data messages in a
retransmit queue until they are acknowledged. These messages are
needed in case MTP3 performs data retrieval as part of a changeover
procedure.
Because propagation delays in IP networks are more variable than in
traditional SS7 networks, a single T7 timer (excessive delay of
acknowledgement), as in MTP2, is inadequate. If any message is
unacknowledged after a period equal to the T7 value, the T7 timer
SHALL expire.
4.2.2. MTP3 Signaling Link Congestion
M2PA SHALL detect transmit congestion in its buffers according to the
requirements for signaling link transmit congestion in MTP3, e.g.,
Q.704 [Q.704], Section 3.8.
4.2.3. Changeover
The objective of the changeover is to ensure that signaling traffic
carried by the unavailable signaling link is diverted to the
alternative signaling link(s) as quickly as possible while avoiding
message loss, duplication, or mis-sequencing. For this purpose, the
changeover procedure includes data retrieval, which is performed
before opening the alternative signaling links to the diverted
traffic. Data retrieval consists of these steps:
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
(1) buffer updating, i.e., identifying all those User Data
messages in the retransmission buffer of the unavailable
signaling link which have not been received by the far end
M2PA, as well as untransmitted messages, and
(2) transferring those messages to the transmission buffers of the
alternate links.
Note that only User Data messages containing data are retrieved and
transmitted over the alternate links. Link Status messages and empty
User Data messages SHALL NOT be retrieved and transmitted over the
alternate links.
M2PA's Sequence Numbers are 24 bits long. MTP2's Forward and
Backward Sequence Numbers are only seven bits long. Hence, it is
necessary for MTP3 to accommodate the larger sequence numbers. This
is done through the use of the Extended Changeover Order (XCO) and
Extended Changeover Acknowledgement (XCA) messages instead of the
Changeover Order (COO) and Changeover Acknowledgement (COA) messages.
The XCO and XCA messages are specified in [Q.2210] Section 9.8.1 and
T1.111.4 [T1.111], Section 15.4. Only the XCO and XCA messages from
[Q.2210] or [T1.111] are required. The BSN is placed in the XCO/XCA
message as explained in [Q.2210] and [T1.111].
Also, the following MTP3/MTP2 primitives MUST use the larger sequence
numbers:
- BSNT Confirmation
- Retrieval Request and FSNC
If M2PA receives a Retrieval Request and FSNC request from MTP3, M2PA
SHALL retrieve from its buffers and deliver to MTP3 in order:
(a) any transmitted User Data messages beginning with the first
unacknowledged message with FSN greater than FSNC.
(b) any untransmitted User Data messages.
For emergency changeover, MTP3 retrieves only the unsent messages for
transmission on the alternate link(s). If M2PA receives a Retrieval
Request and FSNC request with no FSNC value, or with an invalid FSNC,
then M2PA SHALL retrieve from its buffers and deliver to MTP3 in
order:
(a) any untransmitted User Data messages.
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
The Japanese TTC version of MTP defined in [JT-Q703] and [JT-Q704]
has a Retrieval Request (as well as Retrieval Request and FSNC). The
Retrieval allows MTP3 to retrieve both unsent and unacknowledged
messages for transmission on the alternate link(s). In this version
of MTP, if M2PA receives a Retrieval Request, then M2PA SHALL
retrieve from its buffers and deliver to MTP3 in order:
(a) any transmitted but unacknowledged User Data messages.
(b) any untransmitted User Data messages.
4.2.3.1. Multiple User Data Streams and Changeover
The changeover procedure makes it problematic for M2PA to have
multiple User Data streams in one direction for a link. Buffer
updating would have to be done separately for each User Data stream
to avoid duplication or loss of messages. But MTP3 provides for only
one XCO/XCA message for sending the last-received sequence number.
Even with sequence numbering of User Data messages at the M2PA layer,
it is necessary to perform buffer updating on each stream. Since the
M2PA messages would be delivered over multiple streams, there could
be a gap in the M2PA sequence numbers at the receiving end when the
changeover procedure begins. If only the M2PA sequence number is
used in the XCO/XCA message, there would be a possibility of losing
the messages in the gap, or duplicating messages after the gap.
M2PA links with multiple User Data streams would be possible if a
multiple-BSNT XCO/XCA message is defined in MTP3, or if MTP3 allows
multiple XCO/XCA messages (one for each User Data stream) to be sent
during a changeover. This is beyond the scope of this document.
4.3. SCTP Considerations
Some M2PA procedures may be affected by the use of SCTP as a
transport layer. These considerations are discussed in this section.
4.3.1. SCTP Slow Start
SCTP contains a slow start algorithm to control the amount of data
being injected into the network. The algorithm allows SCTP to probe
the network to determine the available capacity. The algorithm is
invoked in these cases: when transmission begins on an association,
after a sufficiently long idle period, or after repairing loss
detected by the SCTP retransmission timer.
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
It is possible that transmission of M2PA messages MAY be delayed by
SCTP slow start under certain conditions, including the following:
(a) Link Alignment. Link alignment takes place after an
association is established. SCTP invokes the slow start
algorithm since transmission is beginning on the association.
(b) Changeover. Messages are retrieved from one link
(association) and transferred to another for transmission. If
the second link had previously been idle, or is in the process
of link alignment, SCTP may invoke the slow start algorithm.
(c) Path failure (multi-homing). If SCTP switches from a failed
path to a new path, and the new path had previously been idle,
SCTP may invoke the slow start algorithm.
(d) Reduced traffic volume. Any time that M2PA sends a low volume
of traffic on a link and then the volume increases, SCTP may
invoke the slow start algorithm.
Programmers should be aware of this condition and how it may affect
M2PA performance. In some cases, it may be possible to avoid the
negative effects of slow start. For example, the Link Status Proving
messages sent during the proving period may be used to complete slow
start before the link is placed in service.
5. Examples of M2PA Procedures
In general, messages passed between MTP3 and M2PA are the same as
those passed between MTP3 and MTP2. M2PA interprets messages from
MTP3 and sends the appropriate message to SCTP. Likewise, messages
from SCTP are used to generate a meaningful message to MTP3.
Note that throughout this section, the primitives between MTP3 and
M2PA are named using the MTP terminology [Q.700] [Q.701] [Q.702]
[Q.703] [Q.704] [Q.705]. Communications between M2PA and SCTP are
named using SCTP terminology.
5.1. Link Initialization (Alignment)
An example of the message flow used to bring an SS7 link in service
is shown in Figures 11 and 12. Alignment is done by both ends of the
link. To simplify the diagram, alignment is shown on one end only.
Some messages from the remote end are not shown. It is assumed in
this example that SCTP has been initialized.
George, et al. Standards Track [Page 34]

RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 20056. Security Considerations
M2PA is designed to carry signaling messages for telephony services.
As such, M2PA MUST involve the security needs of several parties:
- the end users of the services
- the network providers
- the applications involved
Additional requirements MAY come from local regulation.
While these parties may have some overlapping security needs, their
needs may not be identical. Any security solution SHOULD fulfill all
of the different parties' needs.
Consult [RFC3788] for a discussion of security requirements and for
guidance on the use of security protocols. Implementers of M2PA MUST
follow the guidelines in [RFC3788].
7. IANA Considerations7.1. SCTP Payload Protocol Identifier
The SCTP Registered User Port Number Assignment for M2PA is 3565.
The TCP Registered User Port Number 3565 is also assigned to M2PA, in
case a specification for M2PA over TCP is created.
The value assigned by IANA for the Payload Protocol Identifier in the
SCTP Payload Data chunk is
M2PA 5
The SCTP Payload Protocol Identifier is included in each SCTP Data
chunk, to indicate which protocol the SCTP is carrying. This Payload
Protocol Identifier is not directly used by SCTP but may be used by
certain network entities to identify the type of information being
carried in a Data chunk.
The User Adaptation peer may use the Payload Protocol Identifier as a
way of determining additional information about the data being
presented to it by SCTP.
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 20057.2. M2PA Protocol Extensions
This protocol may be extended through IANA in three ways:
- through definition of additional message classes,
- through definition of additional message types, and
- through definition of additional message parameters.
The definition and use of new message classes, types, and parameters
is an integral part of SIGTRAN adaptation layers. Thus, these
extensions are assigned by IANA through an IETF Consensus action as
defined in [RFC2434].
The proposed extension must in no way adversely affect the general
working of the protocol.
The defined values for the message classes, types, and parameters are
listed in the Signaling User Adaptation Layer registry
(sigtran-adapt).
7.2.1. IETF Defined Message Classes
The documentation for a new message class MUST include the following
information:
(a) A long and short name for the message class.
(b) A detailed description of the purpose of the message class.
7.2.2 IETF Defined Message Types
Documentation of the message type MUST contain the following
information:
(a) A long and short name for the new message type.
(b) A detailed description of the structure of the message.
(c) A detailed definition and description of the intended use of
each field within the message.
(d) A detailed procedural description of the use of the new
message type within the operation of the protocol.
(e) A detailed description of error conditions when receiving this
message type.
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
When an implementation receives a message type that it does not
support, it MUST discard the message.
7.2.3. IETF-defined Parameter Extension
Documentation of the message parameter MUST contain the following
information:
(a) Name of the parameter type.
(b) Detailed description of the structure of the parameter field.
(c) Detailed definition of each component of the parameter value.
(d) Detailed description of the intended use of this parameter
type, and an indication of whether, and under what
circumstances, multiple instances of this parameter type may
be found within the same message type.
7.2.4. Defined Values
This section lists the values defined in this document that should be
included in the Signaling User Adaptation Layer registry
(sigtran-adapt).
The following values for Message Class are defined in this document:
Value
(decimal) Message Class
--------- -------------
11 M2PA Messages
The following values for Message Type are defined in this document:
Value
(decimal) Message Type
--------- -------------
1 User Data
2 Link Status
8. Acknowledgements
The authors would like to thank the following for their valuable
comments and suggestions: Brian Tatum, Wayne Davis, Cliff Thomas,
Jeff Copley, Monique Bernard, Malleswar Kalla, Ian Rytina, Greg
Sidebottom, Al Varney, Jeff Craig, and Andrew Booth.
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RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005
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George, et al. Standards Track [Page 53]